Mobile offshore rigs, especially at present, are designed both as, so-called semi-submersible drilling platforms and so-called jack-up drilling rigs, the latter being provided with legs or columns drivable in a vertical direction with respect to the frame structure of a drilling rig for steadying the drilling rig on the seabed in operating condition. Semi-submersible offshore rigs include an underwater section for supporting the actual working platform on the surface. This type of drilling rig is not secured to the seabed at all in a drilling condition, and therefore, such a drilling rig must be provided with expensive and sophisticated articulation and motion compensating mechanisms between a ground drilling mechanism and an offshore rig in order to enable drilling on rough seas. Both manufacturing and operating costs for this particular type of offshore rigs exceed many times those of the above-mentioned jack-up type drilling rigs.
In particular, Finnish patents Nos. 96896 and 100197 disclose solutions, especially for further development of traditional jack-up type drilling platforms. The solution disclosed in the former of these patents is intended for improving safety and usability aspects in an offshore rig in such a way that living quarters, included in the drilling rig, are designed as a movable unit, whereby, in a preferred application, it is removed, at least for the duration of a drilling operation, in a direction opposite to the traveling direction of a drilling unit.
The latter patent offers a solution, which is intended for improving the usability of a jack-up type drilling rig, particularly in reference to the safety of attachment and detachment procedures. Therefore, below the bottom of a frame structure is provided an air space, which is exhaustible for a shipping condition of the drilling rig and which is injected with air for building an air cushion or the like underneath the drilling rig for the duration of the above-mentioned procedures.
At present, offshore operations are still carried out by using prior known maritime units of so-called liftboat type. FIG. 3 shows a few liftboat solutions of this type, which are applicable at various depths and which are particularly intended for providing assistance in all types of offshore processes by having themselves, in the operational condition thereof, steadied or stabilized on the seabed.
Presently, a particular drawback in the above type of maritime units steadied on the seabed is the primitivity of jack mechanisms operating the legs or columns thereof. The reason for this is that these are conventionally designed with highly traditional mechanisms. Such traditional jack mechanisms, capable of sufficiently smooth hoisting, are generally implemented by using rack-and-pinion operated gear assemblies, which are very slow and highly expensive to design because of long racks. On the other hand, there are presently available a number of jack mechanisms driven directly by hydraulic cylinders, but such hydraulic mechanisms, as available at present, require that, as the stroke of each hydraulic cylinder has come to a stop, each movable leg or column be provided with a tenon-and-mortise locking for the duration of returning this particular cylinder to its starting position for the next stroke and the next tenon-and-mortise locking. As a result, traditional hydraulic mechanisms are not capable of stepless operation. Slowness is further enhanced by the fact that the legs or columns are never level with each other on the bottom of the sea. As the cylinders for one leg or column reach the end positions thereof and stop for relocating the tenons, all the rest of the legs come to a stop as well, even though a relocation of the tenons thereof is not yet called for or even desired. Consequently, such mode of operation is extremely tedious and laborious, in addition to which the automation of processes associated with operating the legs or columns is highly inconvenient and expensive, as such a process requires highly sophisticated accessory equipment, particularly for providing a reliable locking action.
Thus, all prior art operating modes are very slow. In addition, the gearshift-based solutions, capable of continuous hoisting action, are extremely expensive in terms of costs.